Jottings . . .

O ne study is rarely definitive, and we accumulate pieces of the puzzle of benefits and harms. In this issue is an interesting pair of articles on long acting b agonists. The first study shows an advantage of self titration with a combination long acting b agonist and inhaled corticosteroid—a revolutionary approach. A second shows possible harms from long acting b agonists. Is it the specific b agonist? Will the combination with corticosteroids prevent the possible harmful effects? The trials answer many questions but raise new uncertainties that we must accept as part of the background of practice. Meanwhile some accepted "truths" have been found wanting: humidifiers for croup and (most) lifestyle changes for reflux do not fare well. If you are interested in a particular topic, the back issues of the EBM journal now form a superb summary of key studies in the last 6 years. If you go to EBM online you can browse by topic. For example, going to "Cardiovascular/Hypertension" brings up 43 articles on hypertension since 2000, discussing, for example, b blockers, the ALLHAT and ASCOT trials, lifestyle changes, and self monitoring. Finally, as of this issue we are trying to expand on the detail provided for the description of interventions. Because many of these are longer than we can accommodate in the paper version they will generally be provided as "web extras"—so watch out for this and get the extra details online.

If you are interested in a particular topic, the back issues of the EBM journal now form a superb summary of key studies in the last 6 years. If you go to EBM online you can browse by topic. For example, going to "Cardiovascular/Hypertension" brings up 43 articles on hypertension since 2000, discussing, for example, b blockers, the ALLHAT and ASCOT trials, lifestyle changes, and self monitoring.
Finally, as of this issue we are trying to expand on the detail provided for the description of interventions. Because many of these are longer than we can accommodate in the paper version they will generally be provided as "web extras"-so watch out for this and get the extra details online.

THE EDITORS
Making sense of diagnostic tests likelihood ratios A smart mother makes often a better diagnosis than a poor doctor. August Bier German, professor of surgery 1861-1949. S tatistical diagnosis is a mystery to many health practitioners. 1 Information is often presented as test accuracy (sensitivity and specificity). However, at the bedside we need to know how a test result predicts the diagnosis or outcome of interest. Attempts to demystify approaches to diagnosis include the construction of 2 by 2 boxes, nomograms, and rules such as SpPin (with a Specific test the Positive Rules in) and SnNout (with a Sensitive test the Negative Rules out). In our teachings of evidence-based medicine, we have found an easier, intuitive way to interpret the results of diagnostic studies based on 2 elements: the likelihood ratio of the test and the pretest odds.
The likelihood ratio (LR) summarises information about the diagnostic test by combining information about the sensitivity and specificity. It tells you how much a positive or negative result changes the likelihood that a patient would have the disease. The LR of a positive test result (+LR) is equal to the sensitivity divided by 1 2 specificity, while the LR of a negative test result (2LR) is 12 sensitivity divided by specificity. Figure 1 illustrates 2 ways of calculating the chances of having a particular condition. The first uses the prevalence of a particular disease to formulate the pretest probability; the LR in the nomogram is then used to calculate the post-test probability. 2 The second way is to convert the pretest probability into pretest odds and then calculate the post-test odds. The graphical cube now represents the calculation undertaken when using the nomogram. We propose that it is simpler to think of diagnostic tests just using the cube in figure 1.
The formula within the cube shows that the pretest odds are modified by the diagnostic information summarised by the LR. The odds increase if the LR is greater than 1, decrease if the LR is less than 1, and stay the same if the LR is 1. Therefore an LR of 5 would increase the initial odds by a factor of 5, while an LR of 0.5 (ie, K) would decrease the odds by half. The box below shows the steps for making sense of diagnostic test: A nomogram is a graphical calculator that is a useful and convenient way to perform calculations without the need to remember formula (Page and Attia 3 ).
Steps for interpreting diagnosis tests